CG position

[Mike downs 1]

I have designed a flying and used a cg finding app called ecalc

it gave me a cg at 3.5” from the le

at full power it climbs steeply

,when I throttle back it levels out

any suggestions greatly appreciated 

[Philip Lewis 3]

You need down thrust on the motor.

[PatMc]

Downthrust is a free-flight/single channel hangover from the days when there was no elevator or proportional throttle control.

 

The app will only give you a ball-park safe cg position, you need to fine tune it for best overall performance.

Move the cg further back incrementally, retrimming the elevator for level flight at cruise throttle setting with each change but stop when you feel that the model is becoming too sensitive to elevator control. 

At this point the model should climb at a moderate angle at full throttle & descend when the throttle is below cruise setting. The practical range of rates of climb & descent will depend on several other factors such as the wing section power to weight ratio etc.

[Peter Jenkins]

1 hour ago, PatMc said:

Downthrust is a free-flight/single channel hangover from the days when there was no elevator or proportional throttle control.

 

The app will only give you a ball-park safe cg position, you need to fine tune it for best overall performance.

Move the cg further back incrementally, retrimming the elevator for level flight at cruise throttle setting with each change but stop when you feel that the model is becoming too sensitive to elevator control. 

At this point the model should climb at a moderate angle at full throttle & descend when the throttle is below cruise setting. The practical range of rates of climb & descent will depend on several other factors such as the wing section power to weight ratio etc.

Have to disagree that downthrust is a hangover from FF and single channel.  I set down and side thrust to give as little change in aircraft attitude when the throttle is opened and closed.  Of course, as the airspeed increases then lift will increase and the aircraft will gently climb after a few seconds and the same when closing the throttle.  What you don't want is what the OP says is an over sensitive reaction to opening and closing the throttle.  

 

You adjust side thrust so that the aircraft isn't dragged off to the left or right as the throttle is opened and closed.  This is most readily seen when pulling to the vertical, to perform a stall turn say, and the aircraft yaws off one way or the other.  Aim to increase side thrust (to the right assuming a tractor configuration) to prevent a yaw to the left and vice versa.

 

Doing that will make flying the aircraft during big power changes much easier to handle.  Of course, you can choose to use the elevator and rudder, perhaps even mixed with the throttle, to overcome these issues, but that only works for one speed as aerodynamic forces vary with the square of the speed.

[PatMc]

3 minutes ago, Peter Jenkins said:

Have to disagree that downthrust is a hangover from FF and single channel.  I set down and side thrust to give as little change in aircraft attitude when the throttle is opened and closed.  Of course, as the airspeed increases then lift will increase and the aircraft will gently climb after a few seconds and the same when closing the throttle.  What you don't want is what the OP says is an over sensitive reaction to opening and closing the throttle.  

 

You adjust side thrust so that the aircraft isn't dragged off to the left or right as the throttle is opened and closed.  This is most readily seen when pulling to the vertical, to perform a stall turn say, and the aircraft yaws off one way or the other.  Aim to increase side thrust (to the right assuming a tractor configuration) to prevent a yaw to the left and vice versa.

 

Doing that will make flying the aircraft during big power changes much easier to handle.  Of course, you can choose to use the elevator and rudder, perhaps even mixed with the throttle, to overcome these issues, but that only works for one speed as aerodynamic forces vary with the square of the speed.

 

Not sure why you bring side-thrust into the discussion, the OP didn't mention it (nor did I).

 

Optimising the CG will reduce the range of variation in climb & descent caused by changes in power to a manageable level without wasting some of that power by pointing it where it's not productive. If it's an aerobatic model any downthrust could be an embarrassment when inverted. 

By optimising the cg you are eliminating a design problem not patching it up.

Using the well known "dive test" is another way of establishing the most suitable cg position but whilst it's the best method to use for gliders it's not really necessary with a power model where varying the throttle setting has the same effect as gravity on a glider.     

[EarlyBird]

7 hours ago, PatMc said:

Downthrust is a free-flight/single channel hangover from the days when there was no elevator or proportional throttle control.

 

The app will only give you a ball-park safe cg position, you need to fine tune it for best overall performance.

Move the cg further back incrementally, retrimming the elevator for level flight at cruise throttle setting with each change but stop when you feel that the model is becoming too sensitive to elevator control. 

At this point the model should climb at a moderate angle at full throttle & descend when the throttle is below cruise setting. The practical range of rates of climb & descent will depend on several other factors such as the wing section power to weight ratio etc.

Agree with this as it's exactly what I do now with the ultimate aim of having the model continue flying straight and level when the throttle is increased from cruise to full throttle. If I get it right it will fly inverted with no elevator input needed. I have been told this is making it so that 'it goes where you point it'. Conversely as a complete novice I was taught to add down thrust to gently climb with increased throttle. Quick easy fix more relaxed flying for a novice?     

[Jonathan M]

I disagree with absolute views.  Without some down-thrust my Wot4, my Boomerang and my old Acrowot (for examples) all of which have very rearward/neutral CGs, will climb dramatically the moment I open the throttle, which is no good for anything approaching precision in aerobatic manoeuvres.  Without a bit of down shimmed in I'd have to lean heavily forward on the stick when speeding up towards show centre for say a loop or stall-turn.  Similarly, without a smidge of right-thrust they'd kick over to the left when the power comes on.  All of this increases pilot workload unnecessarily.  (Maybe if one flies scale warbirds then learning to anticipate and correct for these effects is part of the challenge?)

 

But it surely depends on the actual model type/design and a whole host of interrelated factors and forces.  The OP hasn't been clear about what he's actually designed, but if its a "wing" (the missing word in his opening post?) then it is quite feasible that the rotating prop-wash is in fact having a magnified adverse effect on the pitch of the model when the power comes on?  In which case experimenting with down-thrust - after having played around with CG - might reduce the problem (which in turn will also depend to some extent on the diameter and pitch of the prop)?

 

I build and fly FF scale models:  trimming these is an iterative process which involves isolating and tweaking each element (CG, thrust line, rudder pre-set, wing-tabs, wing-weights, even incidence) one at a time - some of which need re-tuning the moment one installs a rubber motor of thicker/thinner cross-section (power) or shorter/longer length (duration).

[Martin Harris - Moderator]

Isn’t the relationship between the thrust line and the centre of pressure a major factor?

 

Mid wing, nose mounted engine won’t need any, high wing may need a little and a high engine in something like an amphibious will need significant upthrust. 

[EarlyBird]

20 minutes ago, Martin Harris - Moderator said:

Isn’t the relationship between the thrust line and the centre of pressure a major factor?

That's my understanding and as we have not been given any details by the OP it's difficult to make the advice relevant to his specific case.

[Jonathan M]

30 minutes ago, Martin Harris - Moderator said:

Isn’t the relationship between the thrust line and the centre of pressure a major factor?

 

Mid wing, nose mounted engine won’t need any, high wing may need a little and a high engine in something like an amphibious will need significant upthrust. 

Yes.

 

Although a high-wing design, my rubber-powered 16" Pilatus Porter float-plane didn't need any down-thrust at all.

 

My electrified 13" Bristol Scout needed a significant amount of both down- and side-thrust, which was largely a function of its very short nose.

 

That's what I was alluding to in my previous post - too many factors in any given model design for a one-size-fits-all solution.

 

EB is right - for all we know right now the OP has designed a flying carrot-peeler...!

 

 

 

[Engine Doctor]

Mike can you confirm you OP . Is it a flying wing ? Is it a pusher or a tractor motor ?

[Mike downs 1]

22 minutes ago, PatMc said:

Downthrust is a free-flight/single channel hangover from the days when there was no elevator or proportional throttle control.

 

The app will only give you a ball-park safe cg position, you need to fine tune it for best overall performance.

Move the cg further back incrementally, retrimming the elevator for level flight at cruise throttle setting with each change but stop when you feel that the model is becoming too sensitive to elevator control. 

At this point the model should climb at a moderate angle at full throttle & descend when the throttle is below cruise setting. The practical range of rates of climb & descent will depend on several other factors such as the wing section power to weight ratio etc.

 

[Mike downs 1]

It is a flying wing with the motor up front.I could include a picture but I don’t know how to do that.

[Mike downs 1]

[Mike downs 1]

Aah.this is the model that I am referring to.Thanks for all the comments I will implement a change of down thrust first and see if it makes a difference and report back!

[J D 8]

Flying wing will have a reflex wing section or a false reflex from elevator/elevons being set up some. This type tends of aircraft tends to need a more forward CG than the usual wing with tail set up. The drag on that huge fin may also pull the nose up as speed increases. The lack of the word " wing" in the OP lead to confusion in our response to this one.    EB was right, it is a flying carrot peeler.

[John Lee]

I use a down elevator/throttle mix on a couple of my flying wings.

[Jonathan M]

Nice serrations Mike!

 

With that fully exposed motor thrust adjustments should be pretty quick and easy.  If that doesn't help solve things satisfactorily, then - as John Lee suggests - a slight down-elevator to throttle mix would definitely be worth trying.

[Engine Doctor]

I have a similar type of model that looks like a Buzzard. Its great for teasing the crows and occasionally a real buzzard will come over for a look. It needs quite a bit of down thrust. As it's meant to glide quite slow any excess speed causes it to climb steeply. An elevator / throttle mix is fine unless you close throttle sharply before speed is lost , the model zooms up.

Edited November 12, 2021 by Engine Doctor

[Mike downs 1]

8 hours ago, Engine Doctor said:

Mike can you confirm you OP . Is it a flying wing ? Is it a pusher or a tractor motor ?

 

[Mike downs 1]

What is a carrot peeler?

[Ron Gray]

A ginger copper.

Edited November 12, 2021 by Ron Gray

[Mike downs 1]

Still don’t get it…….Have I wandered on to a different thread?

[Don Fry]

I once had the pleasure.

[Mike downs 1]

Knobs